Echo suppressor



Aug. 12, 1930.

p. MITCHELL ET AL EGHO SUPPBESSOR Filed Feb. 2o, 1929 llllllli IIIindicates paling q/all' atta-des,

INVENTORS L/ v ATToNEY Patented Aug. 12, 1930 UNITED STATES -PATENTOFFICE DOREN MITCHELL, 0F NEW YORK, N. Y., AND HAROLD C. SILENT, OF LOSANGELES, CALIFORNIA, ASSIGNORS TO AMERICAN TELEPHONE AND TELEGRAI'HCOMPANY,

A CORPORATION 0F NEW YORK ECHO SUPPRESSOR Application filed FebruaryA20, 1929. Serial No. 341,542.

This invention relates to two-Way transmission circuits, and moreparticularly to four-wire circuits and the apparatus often associatedtherewith for suppressing echoes.

For a discussion of some ot the fundamental problems connected-'With thepresence of echoes in two-way transmission circuits, and for adescription of echo-suppressing devices which are among those betterAknown in the art, reference may be had to an article by A. B, Clark andR. C. Mathes, Journal of the American Institute of Electrical Engineers,June 1925, pages 618-626.

y One object of the invention is to provide for reduction of thesensitivity of the echosuppressor switches in such a manner as toprevent false operation of these devices by noise in the transmissionaths.

A second object of the 1nv'ention is to prevent the above-mentionedfalse operation, Without rendering the echo suppressor insuicientlysensitive to voice waves when no noise is present.

A further object of the invention is the reduction ofthe sensitivity ofthe echo-suppressor apparatus to noise currents without introducingimproper interaction between the two sides of the echo-suppressorsystem.

In attaining the above stated objects, the applicants, in general,provide means responsive to noise in either side of the four-wirecircuit for biasing the echo-suppressor relays, and means for preventingthe biasing of the relays in response to voice Waves, and arrange thecircuits designed to accomplish the control of the echo-suppressorrelays in such a manner that neither side of the system acts falselyupon the other side.

The invention will now be more clearly understood from the followingdetailed description ot' Yone desirable embodiment thereof. Thisdescription is to be read with reference to the accompanying drawing,showing diagrammatically the circuits and apparatus involved.

With reference to the details of the drawing, there is shown a sectionof a four-Wire circuit having a path L1 adapted for transmission fromwest to east, for instance, and a second path L2 adapted fortransmission from east to West. Connected acrcs the line L1 is anamplifier A1 which has its output connected tothe input of a detectorD1. The left hand Winding of a relay R1 is included in the plate circuitof detector D1 and this relay controls other relays which directlyaccomplish the echo-suppressing function. Likewise there are associatedwith the line L2 and amplifier A2, a detector D2, and a relay R2, therelay having its left-hand windingc n- `nected in the plate circuit ofthe detec r. The relay R2 controls other relays which irectly accomplishthe echo-suppressing ful ction. The relays controlled by relays R1 a dR2 are fully described in the above-identified article by Clark andMathes; accordinglylthe apparatus controlled by the two relays in thedetector circuits of the applicants arrangement are merely indicated inthe drawing.

It is, of course, desirable that the echosuppressor apparatus describedabove be as sensitive as possible to voice waves in the four-Wirecircuit. It is, however, equally desirable that the echo-suppressingapparatus be not operated in response to noise in the four-wire circuit.

The applicants add a right-hand winding to each ot' the relays R, andR2, these windings being so connected that current through them opposesthe operating current from the detectors Dl and D2, that is, current inthe right-hand windings biases the relays against operation. To controlthe current through these right-hand windings of relays R1 and R2, andconsequently the sensitivity of the relays, the applicants employ thevacuum tubes VTl, RT, VT2, VTS, and VT4. y A relay R', condensers C1 andand resistances r3, m,

f5, and r6 are connected as disclosed in the drawing. The input of tubeVT1, which is an amplifier tube, is associated with amplifiers A1 and A2through a hybrid coil arrangement HC. The resistances 1", and r2,forming parts of the hybrid coil arrangement, are given such values thatcurrent passing from ampliiier A1, for instance., to the tube VTI, Willset up no false action in the side of the echo- .suppressor systemconnected across line L2 of the four-Wire circuit. It is to b understoodthat any suitable arrangemen known in the art may be substituted for-thehybrid coil arrangement described.

The operation of the applicants circuits `for controlling thesensitivity of relays R1 and R2 will now be considered. If voice wavesact on tube VTI, the tube RT,which is a rectifier tube, is operated, andby reason of the connection shown and the action of condenser Cl andresistance r3 a negative po tential is impressed on the grid of thevaccuin tube VT2, which functions as a blocking tube. Thisnegativepotential serves to reduce the current in the plate circuit ofthe tube VT2 and the condenser C2 begins to ydischarge throughresistances r4 a uil T5. Since the voice Wave is rapidly fluctuating,the discharge of condenser C2 will usually be checked by a decrease ofthe amplitude of the input wave, which causes an increase of platecurrent in the blocking tube of VT2 and a re-charging of condenser C2.However, as the condenser C2 discharges through iesistances r4 and n,the grid of tube VT3 becomes less negative. It will be noted that theplate circuit of this tube VTa includes the aiasing windings of relaysR1 and R2 and the normally closed contact controlled by relay R. Withrelay R unoperated, it is evicent that the driving of the grid of tubeVT3 less negative increases the plate current in the tube and tends tobias relays R1 and R2 against operation by detectors D1 and D2, respecLively. Accordingly, voice waves which represented somewhat sustainedenergy might, through the operation of the applicants system` cause aslight reduction of sensitivity in relays Rl and R2, which reductionis'not desired in response to such waves.

In order to prevent the above-mentioned false reduction of sensitivityin relays R1 and R2, the applicants employ the tube VT4 associated witha resistance re, the condenser C2, and the relay R. inoperative, due tothe negative biasing voltage on the grid. 'Voice Waves of considerableamplitude, however, will drive the grid of the tube considerably lessnegative and the tube will pperate, causing the operation of relay R.This relay Ioperation breaks the circuit through the biasing windings ofrelays Rl and R2, and plate current in tube VT?` will have no effect onthe sensitivity of relays R1 and Rg. l

If, however, the waves coming in from the four-wire circuit are notfluctuating voice waves but relatively steady noise, the condenser C2will discharge slowly and fully through resistances r., and 1'5, andplate cir` rent will tend to How intube VT3 not only while the circuitis troken at relay R through operation of tube VT4 but also after tubeVTl has ceased to operate through restored negative biasing voltage onits grid and the circuit has been re-closed at relay R. Thus it will beseen that while the fluctuating The tube VT4 is normally voice waveshave no effect whatsoever on the sensitivity of the echo-suppressorrelays, noise which is sustained will, after a certain time, cause thefull biasing of the echo-suppressor relayis, and accordingly operationof the echosuppressor apparatus in response to noise is substantiallyprevented.

It will be noted that the biasing windings of relays Rl and Rg are soconnected in the system that noise on either side of the fourwirecircuit causes reduction of sensitivity in both sides of the echosuppressor system, and accordingly false operation df the echosuppressor by' noise is prevented.

While the invention has been described in one embodiment which is deemeddesirable, it is to be understood that this description is merely forthe purpose of illustration and does` not limit the scope of theinvention, which is determined by the appended claims.

'hat is claimed is:

l. In a system for suppressing echoes in a two-way circuitavoice-operated device associated with each side of the circuit, a relaycontrolled thereby and adapted to block the oppositeside of the circuit,and means responsive to noise in either side of the circuit for biasingboth the relays to prevent operation thereof by the noise.

2. In a system for suppressing echoes in a two-way circuit, avoice-operated device associated with each side of the circuit. a relaycontrolled thereby and adapted to block the opposite side of thecircuit, means responsive to noise in either side of the circuit forbiasing both the relays to prevent operation thereof by thenoise, andIneans for preventing the biasing of the relays in response to voicewaves.

3. In a system for suppressing, echoes in a two-way circuit, avoice-operated device associated with each side of the circuit, a relaycontrolled thereby and adapted to block the opposite side of thecircuit, means responsive to noise in either side of the circuit forbiasing both the relays to prevent operation thereof by the noise, meansfor preventing the biasing of the relays in response to voice waves, andmeans for preventing interaction between the two sides of theecho-suppressor system due to the association there? with of the biasingmeans.

In testimony whereof, I have signed m name to this specification this18th day ot' l `ebruary` 1929.

DOREN MITCHELL.

In testimony whereof, I have signed my name to this specification this13th day of

